The present invention relates generally to a method and system for measuring angles. Merely by way of example, the invention has been applied to an angle measurement device useful for determining a rotation angle. In a particular embodiment, a system for measuring rotation angles is utilized in a surveying instrument. However, embodiments of the present invention are not limited to this particular application and may be utilized in a variety of measurement systems.
Angle encoders, such as absolute and incremental angle encoders, have been employed for angular measurements in machine tools, drive technology, automotive electronics, and the like. An example of an angle encoder is a rotating disk used in conjunction with a reading head that is able to detect rotation of the disk. In a particular example of a conventional angle encoder, a light source (e.g., an LED) illuminates a partially transparent disk upon which an opaque pattern fixed to the rotating mechanical element is superimposed. A photodetector (e.g., a photodiode) is positioned on the opposite side of the disk and fixed to the stationary mechanical element. Light passing through the partially transparent disk is modulated by the opaque pattern and, as the angular displacement of the disk changes, the light modulation of the pattern will be detected by the photodetector, thereby providing a measure of angular rotation of the disk.
An angle encoder is described in U.S. Patent Application Publication No. 2005/0072912. In this angle encoder, a rotating plate with two openings for light transmission is mounted between two light sources and a two-dimensional profile sensor. Based on a correlation between a first position associated with light from the first light source and a second position associated with light from the second light source, an absolute value of the rotation angle is calculated. However, this device provides limited angular resolution.
Thus, there is a need in the art for improved methods and systems for measuring angle, for example, rotation angles.